Production of lead shielding bricks



Feb. 7, 1967 s, VAN DEN BROEIK 3,302,442

PRODUCTION OF LEAD SHIELDING BRICKS Filed Jan. 6, 1964 BNvEN-roR \SIDNEV TQNEJT" VAN DE/VBP EK AT'roRNEYS United States Patent 3,302,442 PRODUCTION OF LEAD SHIELDING BRICKS Sidney E. van den Broek, Wickham, England, assignor to Graviner Manufacturing Company Limited, Staines, Middlesex, England, a British company Filed Jan. 6, 1964, Ser. No. 335,911 Claims priority, application Great Britain, Jan. 23, 1960, 2,537/ 60 6 Claims. (Cl. 72-359) This is a continuation-in-part of application Serial No. 83,284, filed January 17, 1961, now abandoned.

This invention relates to the manufacture of lead bricks, in particular lead shielding bricks which are used for providing shielding against nuclear radiation. Except where otherwise indicated, the word lead is used herein to include alloys of lead with small quantities of other metals, such as 4% of antimony.

The normal method of producing such bricks is to cast or die-cast them to approximate shape and then machine each casting to its final limits which are often to close tolerances. Since, however, shielding bricks are frequently shaped to fit together and not merely abut, most of such bricks are each formed with a male surface and a female surface, these for convenience being normally in the form of a prismatic projection and a prismatic recess on opposite sides of the brick. Such bricks are commonly referred to as being of chevron shape. Where such bricks are designed to fit together in the vertical as well as the horizontal direction, it will be evident that each brick has two male surfaces and two female surfaces, or in the case of corner bricks a greater number of male and female surfaces. Whilst it is practicable, although expensive, to machine such male surfaces, it is quite impracticable to machine the female surfaces which therefore must be cast as accurately as possible and finished oif by hand. Furthermore, casting of bricks frequently produces a number of blow holes in the surfaces and although these may be machined away on a male surface, when such holes occur in a female surface, the brick must be rejected or the holes filled by the subsequent addition of further molten lead. If voids or fissures are formed in the cast lead, these, if sufficiently large, can be detected by an irradiation testing process and the brick rejected, but it is very difficult to detect small voids and fissures. Porosity in bricks caused by such voids and fissures either render the bricks unsuitable for shielding purposes or reduce their shielding value. If porosity is to be allowed for, the brick must be made larger than would be required in the case of a non-porous brick. The disadvantages mentioned above may be substantially alleviated by the method of forming bricks according to the present invention.

The present invention provides a lead brick brought to a predetermined contour by pressing in the solid state.

The present invention also provides a method of forming such a brick, and apparatus for performing this method.

The invention will now be particularly described, by way of example only, with reference to the accompanying drawings, in which:

FIGURE 1 is a vertical section through a die for cold forming lead bricks.

FIGURE 2 is a section on the line IIII of FIG- URE 1.

FIGURE 3 is an isometric view of a typical lead brick which may be formed with the apparatus of FIGS. 1 and 2, and

FIGURE 4 is a vertical section through a female punch.

As shown in FIGS. 1 and 2, apparatus for cold forming lead bricks comprises a die 11 formed of hardened tool steel having a circular periphery and a die cavity 12 which matches the cross-section of the lead brick to be formed, this die cavity being of constant cross-section throughout its length. The die is supported within a die support or bolster 13 which is bolted to the bed of a high pressure hydraulic press (not shown).

The upper and lower ends of the die cavity are closed by punches 14 and 15 whose longitudinal surfaces match the internal surface of the die cavity. The operative faces 14a and 15a of the punches are profiled to the shape of the corresponding surfaces of the lead brick to be formed. Thus as shown in the drawings the lower punch has a male surface 1411 designed to form a corresponding female surface in the lower face of the brick whilst the upper punch has a female surface 15a designed to form a corresponding male surface on the upper face of the brick. Alternatively, however, the upper punch can have the male surface and the lower punch the female surface. In fact, this arrangement is preferable since the female punch has a tendency to spread under the applied pressure and such spreading of a moving punch could result in binding of the punch with the walls of the die.

Extreme pressure applied to a female punch can lead to splitting of the punch. This danger can be overcome, as shown in FIGURE 4, but the novel feature of forming the punch of a base part, having an inner end wall normal to the axis of the die, and two or more auxiliary parts each of right-angled triangle cross-section mounted on said wall of the base part and secured thereto by means of screws which permit slight movement of the auxiliary parts along said wall, i.e. in a direction normal to the axis of the die. It will thus be seen that, as pressure builds up to the working surfaces of the die, which surfaces include the inclined surfaces of the auxiliary punch parts, these parts will move transversely of the axis of the die until they are pressed firmly against the walls of the die and against the base part of the punch.

Spew gates are provided for allowing excess lead to be squeezed out of the die along one or more surfaces of at least one of said punches. These spew gates may be formed by grooves 14b in the lower punch 14. However, it is preferred to make the upper punch slightly undersize along at least two opposite sides thereof, preferably the sides adjacent the inclined working surfaces of the punch, so that the lead spew will take the form of thin sheets of lead which emerge from the die along said opposite sides of the upper punch.

The brick formed by the die and punches of FIGURE 1 is shown in FIGURE 3. This is a corner brick of relatively complex shape. The more conventional shape of a brick is chevron shape, that is to say having a V section projection extending across the full width of one end of the brick and a V section recess extending across the full width of the opposite end of the brick.

In the operation of the die, with the lower punch in position, a precast lead blank whose volume is in excess of the volume of the desired finished brick, is placed within the die and after insertion of the upper punch, a very high pressure is applied to the die by the press to cause the lead blank to be transformed into a fluid state in which it can flow into the shape of the finished brick, excess lead being squeezed out through the spew gates.

This transformation into a fluid state is necessary in order for the lead to flow into the relatively intricate shape of the chevron die, and the spew holes must be so dimensioned to prevent the plastic extrusion of the lead from the die whilst the pressure is building up, and so retain all the lead in the die until the fluid state is achieved. Moreover, it has been found that porosity is not eliminated unless this fluid state is achieved.

The lower punch has an enlarged head 14c and the upper punch an enlarged head 150 which define the limit of insertion of the punches into the die and therefore define the size of the finished brick. When the brick has been formed in this way, the upper and lower punches are removed and a similarly shaped brick of slightly smaller cross-section is inserted in the die above the finished brick and the top punch replaced and pressed downward to expel the shaped brick from the bottom of the die. Alternatively, the lower punch can be provided with a very long stem so that after pressing of a brick to shape, the upper punch is removed and the lower punch is raised to expel the brick upwardly. The only further operation required to produce a finished brick is the removal of lead flash.

Whilst the blank to be used for forming the brick preferably conforms very approximately to the internal shape of the die, this is not believed to be necessary and a blank of almost any shape may be placed in the die and pressed to form a shaped brick. The internal surfaces of the die and of the punches are polished to provide a smooth or mirror finish, and substantially the same degree of finish will accordingly be imparted to the surfaces of the shaped brick. Furthermore, the operation of pressing the brick out of the die will even further improve the surface finish on the surfaces of the brick contacted by the die. It is believed that such a high degree of surface finish is impossible to achieve by any of the existing lead shaping methods.

In order to transform the lead into the fluid state referred to above, it has been found that a pressure of about 20 tons per square inch is required for pure lead, whilst a pressure of about 40 tons per square inch is required for lead containing 4% antimony.

As an example of the process, a blank of lead containing 4% antimony and weighing 28 pounds was placed in a die of 4 inches by 2 inches crosssection and 8 inches length. The upper punch was smaller by one millimeter along two opposite sides than the cross-section of the die thus providing two spew gates each one millimeter in width along two opposite sides of the die. The die was closed by inserting the upper punch after which pressure was applied and built up to 40 tons per square inch. At this pressure, the lead was transformed into the fluid state, the excess lead was pressed out of the die and simultaneously the pressure dropped to 35 tons per square inch. The finished brick, after removal from the die, was found to Weigh 26 pounds, and was found to have a density of 0.406 pound per cubic inch. This is in comparison with a density of 0.393 pound per cubic inch in the lead blank.

It will thus be seen that, by the method described above, the density of the brick is made substantially greater than that of a cast brick, in other words, bricks may be formed with substantially zero porosity. Moreover, an increased degree of surface hardness is imparted thereto.

The maximum density of lead stated in textbooks is 0.405 pound per cubic inch, and it will be seen that the lead brick produced in the example given above had a measured density slightly in excess of this figure. For shielding against nuclear radiation, it is important that the lead bricks used should be of this maximum density, and no method has been known hitherto of producing lead shielding bricks having this density.

The densified brick is also a mechanically stronger brick. In a test performed on a tensile test machine, a cast and machined brick fractured at 3.8 tons, whilst a densified brick produced in accordance with this invention did not fracture until the load was increased to 5.5 tons.

It will be evident that bricks having a number of different upper and lower surface profiles may be formed with the apparatus described. Thus by interchanging the two punches as shown, a mirror image of the brick of FIGURE 3 may be formed. Alternatively, other surface profiles may be obtained by providing interchangeable punches and/ or by providing inserts for filling certain of the cavities in the die.

As an additional advantage of the present invention, it will be clear that if the brick has been cast from a blank of known composition (for example 96% lead and 4% antimony) the finished brick must have virtually the same composition. This is not necessarily so in the case of a cast brick which has been machined, since if a disproportionate quantity of lead or antimony exists in the surface of the brick, this being removed by machining, the finished brick will have a different composition.

I claim:

1. Apparatus for the cold pressing of lead bricks which comprises a die whose internal cross-section is constant throughout its length and corresponds to the desired crosssection of a lead brick, an upper male punch having a V section projection at its inner end and a lower female punch having a V section recess therein at its inner end, said female punch comprising a base part having an inner end wall normal to the axis of the die and at least two auxiliary parts each of right-angled triangle section, each said auxiliary part resting on said inner end wall of said base part, and screws securing each said auxiliary part to said base part to permit sideways movement of the auxiliary part as pressure is applied thereto during the pressing operation, the male punch having at least one transverse dimension which is slightly less than the corresponding dimension of the die to form spew gates along opposite sides of the male punch to permit excess lead to be expelled from the die.

2. The method of cold pressing a lead brick which comprises defining a die cavity whose cross-section is constant throughout its length, closing the lower end of the die by two punch surfaces which are inclined to one another to define a V section recess, and a further flat surface forming the base of the V section, placing in said die a lead blank of random shape, closing the upper end of the die by punch surfaces which are inclined to one another to define a V section projection, the outer ends of said upper punch surfaces being spaced from the inner walls of the die to provide spew gates to permit the expulsion of excess lead from the die, moving said upper punch surfaces downwardly to compress the lead blank under a pressure in excess of 20 tons per square inch, and permitting the inclined surfaces of the lower punch to move laterally and at the same time slide on said further flat surface to maintain an unbroken surface closing the lower end of the die and, after the lead has flowed to the internal shape of the die cavity, removing the upper punch surfaces and raising the lower punch surfaces to expel the shaped brick from the die cavity.

3. The method of forming a lead brick for use in building shields against nuclear radiation which comprises pressing a lead blank in the cold state in a closed die betwen a male punch and a female punch, the cavity of said die being defined by wall surfaces shaped to the required configuration of the finished brick, said pressing being carried out at a pressure in excess of twenty tons per square inch, causing the lead to assume the fluid state and to flow to the shape of said die cavity, said pressure being carried out until the porosity of the brick being formed is substantially eliminated and until the density of the lead is increased substantially to the maximum value and thereafter removing the brick from the die.

4. The method of claim 3, wherein the lead blank contains about 4% antimony and the pressure is carried out at about 40 tons per square inch.

5. The method of claim 3, wherein one of the punches is made slightly undersized along two opposite sides thereof and said pressure is carried out until lead spew emerges from said die along said opposite sides of said punch.

6. The method of claim 3, wherein the brick is formed in chevron configuration.

5 6 References Cited by the Examiner 1,234,654 7/ 1917 Gaynor 72473 1 702 278 2/1929 Simons 72377 UNITED TES PAT N STA E TS 3,034,178 5/1962 Cartier 25-102 125,376 4/1872 Brown 72-463 163,656 5 1g75 Hinis 72 359 5 CHARLES W. LANHAM, Primary Examiner. 1,190,696 7/1916 Wilzin 72-473 G. P. CROSBY, Assistant Examiner. 

3. THE METHOD OF FORMING A LEAD BRICK FOR USE IN BUILDING SHIELDS AGAINST NUCLEAR RADIATION WHICH COMPRISES PRESSING A LEAD BLANK IN THE COLD STATE IN A CLOSED DIE BETWEEN A MALE PUNCH AND A FEMALE PUNCH, THE CAVITY OF SAID DIE BEING DEFINED BY WALL SURFACES SHAPED TO THE REQUIRED CONFIGURATION OF THE FINISHED BRICK, SAID PRESSING BEING CARRIED OUT AT A PRESSURE IN EXCESS OF TWENTY TONS PER SQUARE INCH, CAUSING THE LEAD TO ASSUME THE FLUID STATE AND TO FLOW TO THE SHAPE OF SAID DIE CAVITY, SAID PRESSURE BEING CARRIED OUT UNTIL THE POROSITY OF THE BRICK BEING FORMED IN SUBSTANTIALLY ELIMINATED AND UNTIL THE DENSITY OF THE LEAD IS INCREASED SUBSTANTIALLY TO THE MAXIMUM VALUE AND THEREAFTER REMOVING THE BRICK FROM THE DIE. 